Wtórne ryolity nadbałtyckiej części Pomorza Zachodniego

Abstract

SECONDARY RHYOLITES IN THE BALTIC PART OF WEST POMERANIA

Summary

In the Baltic part of the West Pomeranian area, effusive rooks have so far been pierced at Kamień Pomorski and Dźwirzyno. In addition, clastic material of these rocks has been found to occur in sedimentary deposits encountered at Karcino. The clastic rocks of the upper stage of the Rotliegendes are of small thickness, ranging from 1 to 4 metres. These are red or grey-rusty, coarse-grained, badly sorted sandstones, passing at places into conglomerates. The coarse-clastic material is of autochthonous origin. In bore hole Kamień Pomorski IG-l, effusive rocks of the lower stage of the Rotliegendes have been encountered at a depth of 2721.8 m, but not pierced to a depth of 2810.5 m. The effusive rocks are restricted here to two varieties: perlitic - younger rhyolite, and miocropoikilitic - older rhyolite. Mineral composition of these rocks is given in Table 1. At Dźwirzyno, volcanic rocks have been ascertained at a depth of 2530.5 m. After piercing 47.7 m the bore hole was abandoned. Here, the varieties pierced in the bore hole are as follows: perlitic variety - the youngest one, microcrystalline variety - transitional one, and micropoikilitic variety – the oldest one. The chemical composition of the rhyolites from Dźwirzyno and Kamień Pomorski is given in Table 2, and the results of calculations into a, b, c and s parameters, according to A. N. Zawaricki, in Table 3. This was a basis to ascertain that the younger rhyolites from Kamień Pomorski and Dźwirzyno (analyses 1, 2 and 5) and the older rhyolites from the same localities (analyses 3, 6 and 7) can well be correlated. The thickness of the rhyolites found in the Baltic part at West Pomerania is unknown. However, taking into account the thickness of the eruptive series from North-East Mecklenburg, Rügen and Usnam (K. H. Albrecht, K. Goldbecher, 1964; W. Rost, W. Schimanski, 1967) we should expect that it is also considerable. Three kinds of rhyolites have been distinguished in the rocks of the Baltic part of West Pomerania: older, transitional and youngest ones (at Kamień Pomorski the transitional rhyolite is absent). Differences between the rhyolites consist in various character of phenocrysts, in mode of crystallization of rock body and in the degree of hydrothermal alteration. In the older rhyolites from Kamień Pomorski, minerals of inter telluric phase occur mainly in porphyry forms, in the younger ones - in glomeroporphyry form. Observations of glomeroporphyry textures demonstrate that these are a result of clot-like developed rocks of relict consolidation texture, disturbed during the movement of eruptive mass. In general, two types of extremely different rhyolites have been distinguished in the rocks examined: 1- older rhyolite that consists of dacitic rock body and of acid macrocrystals of albite, quartz and orthoclase. Miaroles and asymmetric amygdaloidal forms show that solidification of eruptive mass took place under quiet conditions, without greater displacements. The rock body is completely crystallized and secondarily altered. 2 - younger rhyolite built up of leukorhyolitic rock mass and of middle-basic glomeroporphyroblasts of oligoclase-andesine (albite) and of olivine, as well as of chloritized mafic mineral of columnar habitus. Asymmetric amygdaloidal forms and protoclased feldspar crystals point that crystallization took place during a violent displacement of eruptive mass. Rock mass is feebly crystallized and locally chloritized. The successive phases of eruption took probably place in short time spans. At Kamień Pomorski, the eruption of perlitic rhyolite was preceded by a period of brecciation and of hydrothermal activity. At Dźwirzyno, this period corresponded to the era of successive eruption phase expressed in the form of transitional rhyolite that, apart from a different development of rock body, represents rather the older rhyolite. As concerns chemical composition, the rhyolites under consideration belong to rhyolites-leukorhyolites of sodium and potassium character. Owing to the complete alteration of the rock body the rhyolite examined might be called palaeorhyolites. On the other hand, due to a predominating influence of the secondary alteration - albitization, thus due to a partial homogenization of chemical and mineral composition, volcanic rocks may be termed albite palaeorhyolites. A comparison of the effusive rocks of Rotliegendes age, from the Baltic area of West Pomerania and of North-East Mecklenburg, Rügen and Usnam shows that the rhyolites from Kamień Pomorski and Dźwirzyno may generally correspond to the quartz porphyries and rhyo-dacites from the southern part of Rügen, and from Greifswald. A different development of perlitic forms of rock body in rhyolites points to different conditions of lava consolidation. The rock masses are of similar chemical composition, since no considerable differences exist in the results of chemical analyses of quartz porphyries from Rügen, vitrophyres and rhyo-dacitic porphyry from Greifswald (D. Korich, 1967) on the one hand, and the rhyolites from Kamień Pomorski and Dźwirzyno from the other. In addition, in both regions the rocks underwent also hydrothermal alteration processes, among which albitization no doubt played a considerable role.